Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in an imaging device, and more particularly to an imaging lens which is built in an imaging device mounted in an increasingly compact and high-performance smartphone and mobile phone, an information terminal such as a PDA (Personal Digital Assistant), a game console, PC and a robot, and moreover, a home appliance, a monitoring camera and an automobile with camera function.
Description of the Related Art
In recent years, it becomes common that camera function is mounted in the home appliance, the information terminal equipment, the automobile and public transportation. Furthermore, the image sensor of the imaging device such as the monitoring camera and an on-vehicle camera becomes increasingly compact and large in pixel year after year, and the imaging lens is also required to be compact and to have high performance accordingly.
Demand of wide field of view such as a field of view of 180 degrees or more is increased for the imaging lens used for the monitoring camera and the on-vehicle camera. Furthermore, the brighter imaging lens is demanded in accordance with pixel enhancement.
As a conventional imaging lens aiming for the wide field of view and the high performance, Patent Document 1 (JP 5706584 B1) discloses an imaging lens comprising, in order from an object side, a first lens having positive refractive power, a second lens having negative refractive power, a third lens having the negative refractive power, a fourth lens having negative refractive power, and a fifth lens having the negative refractive power.
However, in lens configurations disclosed in the above-described Patent Document 1, when the wide field of view and low F-number are to be achieved, it is very difficult to correct aberration at a peripheral area, and excellent optical performance can not be obtained.
The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an imaging lens with high resolution which satisfies, in well balance, demand of the wide field of view, the low-profileness and the low F-number and excellently corrects aberrations.
Regarding terms used in the present invention, a convex surface, a concave surface or a plane surface of lens surfaces implies that a shape of the lens surface near an optical axis (paraxial portion), and refractive power implies the refractive power near the optical axis. The pole point implies an off-axial point on an aspheric surface at which a tangential plane intersects the optical axis perpendicularly. The total track length is defined as a distance along the optical axis from an object-side surface of an optical element located closest to the object to an image plane, when thickness of an IR cut filter or a cover glass which may be arranged between the imaging lens and the image plane is regarded as an air.
An imaging lens according to the present invention forms an image of an object on a solid-state image sensor, and comprises in order from an object side to an image side, a first lens having negative refractive power, a second lens, a third lens, a fourth lens and a fifth lens.
The imaging lens having the above-described configuration achieves the wide field of view of an optical system by having the negative refractive power. The second lens suppresses a light ray incident angle to the third lens to be small and properly corrects astigmatism and field curvature. The third lens maintains the low-profileness and properly corrects distortion and the astigmatism. The fourth lens maintains the low-profileness and properly corrects spherical aberration and chromatic aberration. The fifth lens properly corrects the chromatic aberration, the distortion, the astigmatism and the field curvature.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (1) is satisfied:0.1<T2/T3<1.16  (1)
where
T2: distance along an optical axis from an image-side surface of the second lens to an object-side surface of the third lens, and
T3: distance along an optical axis from an image-side surface of the third lens to an object-side surface of the fourth lens.
The conditional expression (1) defines a ratio of an interval between the second lens and the third lens to an interval between the third lens and the fourth lens, and is a condition for achieving the low-profileness and the proper aberration correction. By satisfying the conditional expression (1), difference between the interval of the second lens and the third lens and the interval of the third lens and the fourth lens is suppressed from being increased, and the low-profileness is achieved. Furthermore, by satisfying the conditional expression (1), the third lens is arranged at an optimum position, and aberration correction function of the lens becomes more effective.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (2) is satisfied:9<r3/r4<20  (2)where
r3: paraxial curvature radius of an object-side surface of the second lens, and
r4: paraxial curvature radius of an image-side surface of the second lens.
The conditional expression (2) defines relationship between paraxial curvature radii of the object-side surface and the image-side surface of the second lens, and is a condition for properly correcting the aberrations and for reducing sensitivity to manufacturing error. When a value is below the upper limit of the conditional expression (2), the refractive power of the image-side surface of the second lens is maintained, astigmatism and distortion occurred at this surface are suppressed, and it is facilitated to reduce the sensitivity to the manufacturing error. On the other hand, when the value is above the lower limit of the conditional expression (2), the field curvature is properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (3) is satisfied:−0.1<r5/r6<1.4  (3)where
r5: paraxial curvature radius of an object-side surface of the third lens, and
r6: paraxial curvature radius of an image-side surface of the third lens.
When a value is below the upper limit of the conditional expression (3), the refractive power of the third lens becomes appropriate, and the low-profileness is achieved. On the other hand, when the value is above the lower limit of the conditional expression (3), the distortion is properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (4) is satisfied:−1.85<(r9+r10)/(r9−r10)<5.00  (4)
where
r9: paraxial curvature radius of an object-side surface of the fifth lens, and
r10: paraxial curvature radius of an image-side surface of the fifth lens.
The conditional expression (4) defines a shape of the fifth lens, and a condition for securing back focus, achieving the low-profileness and properly correcting the aberrations. By satisfying the conditional expression (4), the low-profileness is facilitated while securing the back focus, and the distortion, the chromatic aberration, the astigmatism and the field curvature are properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that the refractive power of the second lens is negative, and more preferable that a below conditional expression (5) is satisfied:−5.1<f2/f<−2.65  (5)where
f2: focal length of the second lens, and
f: focal length of the overall optical system of the imaging lens.
By having the negative refractive power, the second lens achieves the wide field of view, and properly corrects the astigmatism and coma aberration. The conditional expression (5) defines the refractive power of the second lens, and is a condition for achieving the low-profileness and proper correction of the aberrations. When a value is below the upper limit of the conditional expression (5), the negative refractive power of the second lens becomes appropriate, and the low-profileness is achieved. On the other hand, when the value is above the lower limit of the conditional expression (5), the field curvature is properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that the fourth lens has a biconvex shape having convex surfaces facing both surfaces near an optical axis.
By having the biconvex shape, the object-side surface and the image-side surface of the fourth lens have positive refractive power, and the low-profileness is facilitated. Furthermore, the biconvex shape has an effect to suppress curvature from being large, and to reduce the sensitivity to the manufacturing error.
According to the imaging lens having the above-described configuration, it is preferable that an image-side surface of the fifth lens is the concave surface facing the image side near the optical axis. Furthermore, it is more preferable that an aspheric surface having an off-axial pole point is provided.
When the image-side surface of the fifth lens is the concave surface facing the image side near the optical axis, the field curvature and the distortion are properly corrected. Furthermore, by having the off-axial pole point on the image-side surface of the fifth lens, the field curvature and the distortion are properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that composite refractive power of the first lens, the second lens and the third lens is negative, and more preferable that a below conditional expression (6) is satisfied:−3.5<f123/f<−1.0  (6)
where
f123: composite focal length of the first lens, the second lens and the third lens, and
f: focal length of the overall optical system of the imaging lens.
When the composite refractive power of the first lens, the second lens and the third lens is negative, the wide field of view is more facilitated. The conditional expression (6) defines a range of the composite focal length of the first lens, the second lens and the third lens to the focal length of the overall optical system of the imaging lens, and a condition for achieving the wide field of view and the low-profileness, and the proper aberration corrections. When a value is below the upper limit of the conditional expression (6), the negative composite refractive power of the first lens, the second lens and the third lens becomes appropriate, and correction of the spherical aberration and the distortion becomes facilitated. Furthermore, the low-profileness can be also achieved. On the other hand, when the value is above the lower limit of the conditional expression (6), the wide field of view can be achieved.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (7) is satisfied:−9.5<f1/f<−2.5  (7)where
f1: focal length of the first lens, and
f: focal length of the overall optical system of the imaging lens.
The conditional expression (7) defines the refractive power of the first lens, and is a condition for achieving the wide field of view and the low-profileness, and the proper aberration corrections. When a value is below the upper limit of the conditional expression (7), the negative refractive power of the first lens becomes appropriate and the correction of the spherical aberration is facilitated. On the other hand, when the value is above the lower limit of the conditional expression (7), the wide field of view can be achieved.
According to the imaging lens having the above-described configuration, it is preferable that the refractive power of the third lens is positive, and more preferable that a below conditional expression (8) is satisfied:5.7<f3/f  (8)where
f3: focal length of the third lens, and
f: focal length of the overall optical system of the imaging lens.
When the third lens has the positive refractive power, the low-profileness is more facilitated. The conditional expression (8) defines the refractive power of the third lens, and is a condition for achieving the low-profileness and the proper aberration corrections. When a value is above the lower limit of the conditional expression (8), a total track length is shortened and the coma aberration and the astigmatism are properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (9) is satisfied:14.5<|f5|/f  (9)where
f5: focal length of the fifth lens, and
f: focal length of the overall optical system of the imaging lens.
The conditional expression (9) defines the refractive power of the fifth lens, and is a condition for achieving the low-profileness and proper correction of the aberrations. When a value is above the lower limit of the conditional expression (9), the total track length is shortened and the chromatic aberration and the field curvature are properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (10) is satisfied:1.85<r1/r2<5.75  (10)where
r1: paraxial curvature radius of an object-side surface of the first lens, and
r2: paraxial curvature radius of an image-side surface of the first lens.
The conditional expression (10) defines relationship between paraxial curvature radii of the object-side surface and the image-side surface of the first lens, and is a condition for achieving the proper aberration corrections. When a value is below the upper limit of the conditional expression (10), the astigmatism is properly corrected. When the value is above the lower limit of the conditional expression (10), the field curvature is properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (11) is satisfied:−1.85<r7/r8<−0.55  (11)where
r7: paraxial curvature radius of an object-side surface of the fourth lens, and
r8: paraxial curvature radius of an image-side surface of the fourth lens.
The conditional expression (11) defines relationship between paraxial curvature radii of the object-side surface and the image-side surface of the fourth lens, and is a condition for properly correcting the aberrations and for reducing the sensitivity to manufacturing error. By satisfying the conditional expression (11), the refractive power of the object-side surface and the image-side surface is suppressed from being excessive, and the proper correction of the aberrations is achieved. Furthermore, the sensitivity to the manufacturing error of the fourth lens is reduced.
According to the imaging lens of the above-described configuration, it is preferable that a below conditional expression (12) is satisfied:28<vd4−vd5<78  (12)where
vd4: abbe number at d-ray of the fourth lens, and
vd5: abbe number at d-ray of the fifth lens.
The conditional expression (12) defines relationship between the abbe numbers at d-ray of the fourth lens and the fifth lens, and is a condition for properly correcting the chromatic aberration. By satisfying the conditional expression (12), the chromatic aberration is properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (13) is satisfied:Fno≤2.4  (13)where
Fno: F-number.
The conditional expression (13) defines the F-number. When a value is below the upper limit of the conditional expression (13), brightness demanded for the imaging lens in recent years can be fully secured, when it is mounted in a portable mobile device, a digital camera, a monitoring camera, or an onboard camera.
According to the imaging lens having the above-described configuration, it is preferable that the object-side surface of the second lens is the convex surface facing the object side near the optical axis.
When the object-side surface of the second lens is the convex surface facing the object side near the optical axis, the coma aberration and the field curvature are properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (14) is satisfied:−186<(1−N3)/(r6×f)×1000<8  (14)
where
N3: refractive index at d-ray of the third lens,
r6: paraxial curvature radius of an image-side surface of the third lens, and
f: focal length of an overall optical system of the imaging lens.
The conditional expression (14) defines an appropriate range of the refractive power of the image-side surface of the third lens, and is a condition for reducing the sensitivity to the manufacturing error and properly correcting the aberrations. By satisfying the conditional expression (14), the refractive power of the image-side surface of the third lens becomes appropriate, and the spherical aberration occurred at the third lens can be effectively suppressed and the sensitivity to the manufacturing error is reduced.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (15) is satisfied:−5<(N5−1)/(r9 ×f)×1000<100  (15)where
N5: refractive index at d-ray of the fifth lens,
r9: paraxial curvature radius of an object-side surface of the fifth lens, and
f: focal length of an overall optical system of the imaging lens.
The conditional expression (15) defines an appropriate range of the refractive power of the object-side surface of the fifth lens, and is a condition for reducing the sensitivity to the manufacturing error and properly correcting the aberrations. By satisfying the conditional expression (15), the refractive power of the object-side surface of the fifth lens becomes appropriate, and the spherical aberration occurred at the fifth lens can be effectively suppressed and the sensitivity to the manufacturing error is reduced.
According to the imaging lens having the above-described configuration, it is preferable that the refractive power of the fourth lens is positive, and more preferable that a below conditional expression (16) is satisfied:1.0<f4/f<3.5  (16)where
f4: focal length of the fourth lens, and
f: focal length of the overall optical system of the imaging lens.
When the fourth lens has the positive refractive power, the low-profileness is more facilitated. Furthermore, the conditional expression (16) defines the refractive power of the fourth lens, and is a condition for achieving the low-profileness and the proper aberration correction. When a value is below the upper limit of the conditional expression (16), the positive refractive power of the fourth lens becomes appropriate, and the low-profileness can be achieved. On the other hand, when the value is above the lower limit of the conditional expression (16), the spherical aberration and the coma aberration are properly corrected.
According to the imaging lens having the above-described configuration, it is preferable that a below conditional expression (17) is satisfied:1.55<bf/f<3.20  (17)
where
bf: distance along an optical axis from an image-side surface of the fifth lens to an image plane (namely, back focus), and
f: focal length of an overall optical system of the imaging lens.
The conditional expression (17) is a condition for securing the back focus and achieving the low-profileness. When a value is below the upper limit of the conditional expression (17), the low-profileness can be achieved. On the other hand, when the value is above the lower limit of the conditional expression (17), securing the back focus is facilitated.